Coating of particulate materials is applied within various industries. Thus, for instance, in the pharmaceutical industry drug-containing tablets and pellets are provided with a coating. Thereby it may be achieved that the active drug is released delayed after administration. This makes it possible to have the active substance released in specific areas of the intestine or to achieve a prolonged release of the active substance.
In the detergent industry an enzyme layer may be provided on a carrying core of extender, which layer is protected against oxidation and attrition by a coating. Also fertilizers, plant protecting agents, and several other chemical products are coated. I.e. this invention is relevant for a variety of industrial sectors, e.g. pharmaceutical, food, dairy, chemical, and ingredient.
Special plants and apparatuses have been developed for coating purposes. One embodiment of these prior art apparatuses comprises a cylindrical or conical reaction chamber having a perforated base plate and a spray nozzle in the top of the reaction chamber. By means of air blown through the base plate a fluidized layer of particles is maintained on said plate and the particles are wetted by the coating liquid which is atomized through the spray nozzle. To achieve the desired degree of encapsulation, uniformity and thickness of the coating layer it is necessary to circulate the batch of particles to be coated for a period between minutes and several hours. This type of apparatus has the drawback that the production capacity is small, and agglomeration of the particles is often experienced.
In an improved version of such apparatus (confer for instance WO93/08923) important constructive amendments have been made. The spray nozzle is placed in the centre of the base plate, and spraying coating liquid upwards in the direction of the main streams. Around the spray nozzle the base plate may have an annular area of a higher perforation due to several larger apertures placed close to each other.
The area more distant from the nozzle may be perforated only to a minor degree; it has less and smaller apertures. Above the spray nozzle a vertical Wurster tube is mounted in such a way that a passage remains between the end of the pipe and the base plate. The diameter of the Wurster tube may correspond to the diameter of the area having more pronounced perforation.
Thereby an annular down-flow bed is formed in the reaction chamber between the wall of the reaction chamber and the Wurster tube, and a coating zone is formed in the Wurster tube. The outer annular portion of the base plate is the bottom of the down-flow bed. Due to the only moderate perforation in the outer annular area, the amount of air penetrating the base plate in this section is smaller than the one passing through the interior portion below the coating pipe. Therefore, the column of particles which collects in the down-flow bed zone is only aereated by the weaker air-stream coming from below. On the contrary the stronger stream of air entering the coating pipe carries the particles coming from the down-flow bed upwards through the cloud of atomized droplets.
To reduce the risk for blocking an apparatus has been developed having no coating pipe but a special spray nozzle (cf. EP-A-563402). By means of pressurized air the material to be coated is blown out from the centre of the spray nozzle. Concentric in relation to the central opening of the nozzle is provided an annular nozzle slot for delivering the coating liquid and pressurized air in an inward direction. This design substantially removes the risk for blocking and sticking of the particles to the inner wall of the reaction chamber. However, the capacity and product quality is still far from satisfactory. Besides, this apparatus is only suitable for treating a limited range of products.
WO95/20432 (Aeromatic-Fielder AG) discloses an apparatus for coating discrete solid particles in a coating unit. Each coating unit comprises an upwardly directed nozzle placed in the bottom, an annular opening around the nozzle, a vertical Wurster tube, above the opening but spaced therefrom, and gas guiding walls below the annular opening for accelerating a gas stream flowing towards said opening. Between the guiding walls means are arranged for imparting a swirling flow to said gas stream. Thereby the particles are coated while being conveyed in a swirling gas flow. While the coating quality and the ease of operation are improved, room for improvement as to further minimize the risk of agglomeration and to increase the yield is still possible.
Apparatuses having a nozzle positioned in the side wall of the fluid bed have been suggested in DE 38 06 537 and DE 197 09 589. DE 197 09 589 relates to a fluid bed in which the spray nozzles extend into the fluid bed chamber. DE 38 06 537 discloses a fluidised-bed apparatus having a three-fluid nozzle arranged in the side wall of a fluid bed. The third fluid is a pressurized gas liberated around a two-fluid nozzle through a small annular slot. The latter technology has the drawback that the attrition is high, which results in a low yield.
The present invention is directed to the technical aim of providing an apparatus and a process suitable for preparing coated particles having a smooth surface and a low tendency to form agglomerates, i.e. a high degree of single coated particles. A further object of the present invention is to provide a process which results in a high yield.